Allogeneic stem cell transplantation (alloSCT) can cure hematologic malignancies and nonmalignant disorders of hematopoiesis. Allograft T cells are critical for T cell reconstitution and mediate a potent anti- leukemia/lymphoma effect. Unfortunately donor T cells can broadly attack the recipient causing graft-vs-host disease (GVHD). Methods of promoting the positive effects of donor T cells while minimizing GVHD have been elusive. Much GVHD research has focused on how antigen presenting cells (APCs) initially prime donor T cells in secondary lymphoid tissues (SLTs). However, APCs are not limited to SLTs. Tissue infiltrative APCs (t-APCs) that are resident and are recruited during inflammation play important roles in anti-pathogen and autoimmune responses. t-APCs are heterogeneous in function and ontogeny, differ from organ to organ, and with inflammation. They interact with T cells in situ and can traffic to draining lymph nodes where they can prime T cells or transfer antigen to other APCs. While T cells are required for GVHD, cellular infiltrates in organs with GVHD have abundant MHCII+ t-APCs. Consistent with a role for t-APCs in GVHD, CD4 cells can mediate GVHD indirectly, without making TCR:MHCII contacts with target tissues; histology in these mice reveal CD4 cells adjacent to MHCII+ hematopoietic cells. We hypothesize that T cells in GVHD lesions are activated locally by t-APCs and that conversely t-APCs are activated by donor T cells. In support of this hypothesis we have employed multiphoton intravital microscopy (MPIM) of GVHD tissues in living mice to demonstrate that donor CD4+ and CD8+ T cells make stable interactions with donor-derived CD11c+ or LysM+ cells. A detailed understanding of the interactions between T cells and t-APCs should identify new targets for suppressing GVHD. We propose to investigate t-APCs in GVHD using novel fluorescent protein-transgenic mice, bone marrow from mice in which t-APC subsets can be inducibly deleted and MPIM. Specifically we will: 1) define the identities of t-APCs in GVHD and determine how those cells are recruited; 2) test the hypothesis that T cells make antigen-specific stable interactions with t-APCs in skin and bowel affected by GVHD; and 3) determine whether these interactions contribute to GVHD.